The estrogen receptor from the human uterus (endometrium and myometrium) are being studied by sucrose gradient centrifugation analysis and gel chromatography under various experimental conditions of pH, salt concentration and temperature. The transformation of the cytoplasmic 8 S estrogen receptor into the nuclear 4-5 S estrogen receptor and the factor(s) involved are being investigated in the human uterus. The molecular characteristics of the human uterine estrogen receptor (cytoplasmic and nuclear) are being compared to the data obtained from the rat uterine model. A better understanding of the estrogen regulatory mechanisms will contribute to the development of more effective control of normal uterine growth and reproductive function.